Understanding the mechanisms of regulation of cellular proliferation and differentiation is basic to understanding development of multicellular organisms. One approach to investigating these regulatory mechanisms is to study the behavior of transformed cells. Through the use of Ad2- (non-oncogenic) and SV40- (highly oncogenic) transformed hamster cells, we have identified the phenotypic characteristics of these cells (e.g., Ad2 sensitivity and SV40 resistance to in vitro lysis by non-specific immune effector cells) that correlate with their ability to form tumors in immunocompetent hamsters. We also find that while Ad2-and SV40-transformed cells are equally active in production of transforming growth factors, only SV40-transformed cells secrete a powerful anti-mitogen, which may inhibit host defenses in vivo. We are also using SV40 to study the genetic basis of viral issue tropism. We find that subcutaneously injected small t-antigen mutants of SV40 often induce abdominal B-cell lymphomas in hamsters, rather than the subcutaneous fibrosarcomas induced by wild-type SV40. The mutants may fail to produce a growth factor required for the in vivo transformation of non-proliferating cells. In another project, we are studying the mechanisms of mutagenesis, using an SV40-based shuttle vector as a probe to investigate the molecular mechanisms by which agents that damage DNA induce mutations in mammalian cells. Our studies on replication of UV-damaged SV40 DNA in vivo have led to a well-defined model of how the mammalian cell replication machinery responds to DNA damage, and at what steps in the replication process mutations become irreversibly established. Through use of the shuttle vector, we have extensively characterized the types of mutations that occur in mammalian cells either spontaneously or in response to DNA damage. Analysis of the sequence specificity of these mutations has led to a model which explains how the mammalian DNA polymerase introduces errors during DNA synthesis, causing mutations. Further studies with the vector in an in vitro DNA replication system should allow a correlation with the in vivo replication defects we observed.